This invention relates to an electric motor for a motor-driven compressor. The present invention further pertains to a motor-driven compressor.
Japanese Laid-Open Patent Publication No. 2006-180576 discloses a hermetic compressor including an electric motor section, which is a permanent magnet electric motor, located in a hermetic case, and a rotary compression section, which is rotated by the permanent magnet electric motor via a crankshaft. The permanent magnet electric motor includes a stator on which magnetic pole teeth for coils are formed, and a permanent magnet rotor arranged outward of the stator. The permanent magnet rotor includes a rotor core located on the outer circumference, and a resin bonded permanent magnet mounted on the inner circumferential portion of the rotor core by injection molding. The crankshaft is fixed to the rotor core. Thus, the stator and the rotor of the permanent magnet electric motor are respectively arranged inward and outward of each other. Also, the hermetic compressor is a motor-driven compressor that integrally includes the compression section and the electric motor section.
When the coil of the stator is energized, current flows across a magnetic flux formed by the permanent magnet located on the inner circumference of the rotor core. Then, the permanent magnet rotor and the crankshaft fixed to the permanent magnet rotor are rotated. As the crankshaft rotates, a refrigerant gas drawn in through a suction pipe provided in the hermetic case is compressed in the compression section and discharged to the outside of the hermetic case through a discharge pipe.
According to the technique disclosed in Japanese Laid-Open Patent Publication No. 2006-180576, permanent magnets are provided on the inner circumferential portion of the rotor core, and the permanent magnets are arranged such that the north poles and the south poles of the permanent magnets are alternately arranged in the circumferential direction of the rotor. For example, as shown in FIG. 12 of the present application, permanent magnets 71 have a magnet arrangement of a radial orientation. According to this magnet arrangement, first magnets 71a, the inner circumferential surface of each of which is magnetized to the north pole and the outer circumferential surface of each of which is magnetized to the south pole, and second magnets 71b, the inner circumferential surface of each of which is magnetized to the south pole and the outer circumferential surface of each of which is magnetized to the north pole, are arranged alternately in the circumferential direction. In this case, the magnetic flux exerted from each north pole located on the inner circumference passes inside the stator core 73 arranged inward of the permanent magnets 71 with a gap formed in between. The magnetic flux that has passed inside the stator core 73 forms flow of magnetic flux, or magnetic lines of force toward the south poles adjacent to the north pole. In the permanent magnets 71, the flow of magnetic flux from the south poles located on the inner circumference of the second magnets 71b toward the north poles located on the outer circumference, and the flow of magnetic flux from the south poles located on the outer circumference of the first magnets 71a toward the north poles located on the inner circumference are formed. In the rotor core 72, the flow of magnetic flux from the north poles to the south poles is formed. In this manner, a loop of the magnetic flux, or a magnetic circuit, is formed as the magnetic flux passes inside the rotor core 72. Thus, for example, in a case in which the rotor core 72 is not formed of a ferromagnetic body such as an iron material, motor characteristics, for example, the torque characteristics might deteriorate as the magnetic characteristics of the permanent magnet electric motor change.
In the motor-driven compressor, in which the compression section and the electric motor section are integrated, a balancing weight is provided on, for example, the rotary shaft to cope with the load fluctuation in the compression section. However, in the case in which the balancing weight is provided, a space for providing the balancing weight is required. Therefore, the size of the motor-driven compressor is increased, which might reduce ease of mounting the motor-driven compressor to, for example, a vehicle.
To limit the increase in the size of the motor-driven compressor, there have been attempts to obtain the rotational balance of the motor-driven compressor by changing the shape of the rotor core in the electric motor section. However, when the shape of the rotor core 72 is changed, the following deficiency might occur. That is, in a case in which the shape of the rotor core 72 is changed by forming holes in the rotor core 72, magnetic flux flow is not formed in the holes, and the magnetic characteristics of the permanent magnet electric motor undesirably change. Thus, the required torque is not obtained. In a case in which the thickness of the rotor core is partly changed, the magnetic characteristics undesirably change, and the same problem occurs.
Accordingly, it is an objective of the present invention to provide an electric motor for a motor-driven compressor that is capable of adjusting the rotational balance in the electric motor while inhibiting increase in the size of the motor-driven compressor.